Momento Classic is no longer available for purchase on the App Store. The app was discontinued in June 2016, 9 months after Momento 3 was released.
If you previously purchased Momento then you can download the last available update from the App Store by visiting the 'Updates' tab of the App Store app and then tapping on the 'Purchased' button at the top. You must be logged in with the same Apple ID you used to originally purchase Momento Classic.
If you already have Momento Classic installed but you're not seeing the latest update (2.9.X) then you will need to backup your Momento Classic data, transfer the backup to a safe location, delete the app and re-download it.